1. Field of the Invention
The disclosure is related to a structure of composite crucibles and a high temperature adiabatic method in an arc heating process thereof, and more particularly related to a structure of composite crucibles used in an arc heating process.
2. Descriptions of the Related Art
Arc melting is a process of directly heating and melting metals by an arc in a furnace in vacuum, which mainly generates arc by ionizing noble gas. In order to stabilize the arc, in general, a direct current (DC) is supplied. Furnaces can be categorized into self-consumption furnaces and non-consumption furnaces by whether the furnaces are consumed (melted) or not, and self-consumption furnaces are used in most of the industrial applications.
Since a great amount of heat is generated during the arc melting, and the temperature can be raised to higher than 3600° C., which is higher than the melting points of normal available materials such as copper molds, which can transfer heat rapidly, used during the conventional melting process, the heat is dissipated by water-cooling. Otherwise, the crucibles used in melting will be melted and cause undesired dangerous situations.
The above mentioned melting, cooling and molding technics of the arc cooperating with water-cooled copper molds have the advantage of low contamination to the materials being melted, while being associated with the disadvantage of the generation of the thermal stress that could be strong enough to result in cracking. Such thermal stress is generated since the melting materials have been closely in contact with the water-cooled copper molds and the melting materials have a large gradient in temperature. In addition, regarding the melting materials having a melting point higher than 3000° C., the melting, cooling and molding technics of the arc cooperating with water-cooled copper molds will be subject to the phenomena that the materials can hardly be melted, which also makes melting more difficult.
Therefore, in order to overcome the above problem in arc melting process, one can dispose one or more graphite crucible above the water-cooled copper mold. Graphite is one of the light elements that can withstand a high temperature, and the melting point of graphite is 3850° C. In addition, electrical conductivity and thermal conductivity of graphite are comparable to metals, which are 4 times greater than stainless steel, 2 times greater than carbon steel, 3-3.5 times greater than lead, and about 100 times greater than other metals. However, when the temperature is higher, the thermal conductivity is decreased, which may cause the material to enter into an adiabatic condition under an extremely high temperature. Thus, using graphite crucibles can reduce heat loss and thermal insulation can be improved, as compared with water-cooled copper molds. Further, the materials having a melting point higher than 3000° C. can still be used in the melting processes.